We document that within-individual variation in food choices is substantial and has potentially important consequences for nutrition, and hence well-being. We develop an approach that allows us to study the determinants of this within-individual variation within an economic framework and allow for across-individual preference heterogeneity. We show that around one-fifth of within-individual fluctuations in diet quality is explained by standard economic variables (prices and budgets), along with advertising and weather. The residual fluctuations are important and are larger for lower income and younger people, and individuals who state they are impulsive. We propose a two-selves model of food purchase behavior to structurally interpret these empirical patterns. We use nonparametric revealed preference techniques to show that this model rationalizes our food purchase data.